How to Use C's volatile Keyword

The proper use of C's volatile keyword is poorly understood by many programmers. This is not surprising, as most C texts dismiss it in a sentence or two. This article will teach you the proper way to do it.

Have you experienced any of the following in your C or C++ embedded code?

Code that works fine--until you enable compiler optimizations

Code that works fine--until interrupts are enabled

Flaky hardware drivers

RTOS tasks that work fine in isolation--until some other task is spawned

If you answered yes to any of the above, it's likely that you didn't use the C keyword volatile. You aren't alone. The use of volatile is poorly understood by many programmers. Unfortunately, most books about the C programming language dismiss volatile in a sentence or two.

C's volatile keyword is a qualifier that is applied to a variable when it is declared. It tells the compiler that the value of the variable may change at any time--without any action being taken by the code the compiler finds nearby. The implications of this are quite serious. However, before we examine them, let's take a look at the syntax.

Syntax of C's volatile Keyword

To declare a variable volatile, include the keyword volatile before or after the data type in the variable definition. For instance both of these declarations will declare foo to be a volatile integer:

volatile int foo;
int volatile foo;

Now, it turns out that pointers to volatile variables are very common, especially with memory-mapped I/O registers. Both of these declarations declare pReg to be a pointer to a volatile unsigned 8-bit integer:

volatile uint8_t * pReg;
uint8_t volatile * pReg;

Volatile pointers to non-volatile data are very rare (I think I've used them once), but I'd better go ahead and give you the syntax:

int * volatile p;

And just for completeness, if you really must have a volatile pointer to a volatile variable, you'd write:

int volatile * volatile p;

Incidentally, for a great explanation of why you have a choice of where to place volatile and why you should place it after the data type (for example, int volatile * foo), read Dan Sak's column "Top-Level cv-Qualifiers in Function Parameters" (Embedded Systems Programming, February 2000, p. 63).

Finally, if you apply volatile to a struct or union, the entire contents of the struct/union are volatile. If you don't want this behavior, you can apply the volatile qualifier to the individual members of the struct/union.

Proper Use of C's volatile Keyword

A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change:

Peripheral Registers

Embedded systems contain real hardware, usually with sophisticated peripherals. These peripherals contain registers whose values may change asynchronously to the program flow. As a very simple example, consider an 8-bit status register that is memory mapped at address 0x1234. It is required that you poll the status register until it becomes non-zero. The naive and incorrect implementation is as follows:

This will almost certainly fail as soon as you turn compiler optimization on, since the compiler will generate assembly language that looks something like this:

mov ptr, #0x1234 mov a, @ptr
loop:
bz loop

The rationale of the optimizer is quite simple: having already read the variable's value into the accumulator (on the second line of assembly), there is no need to reread it, since the value will always be the same. Thus, in the third line, we end up with an infinite loop. To force the compiler to do what we want, we modify the declaration to:

uint8_t volatile * pReg = (uint8_t volatile *) 0x1234;

The assembly language now looks like this:

mov ptr, #0x1234
loop:
mov a, @ptr
bz loop

The desired behavior is achieved.

Subtler problems tend to arise with registers that have special properties. For instance, a lot of peripherals contain registers that are cleared simply by reading them. Extra (or fewer) reads than you are intending can cause quite unexpected results in these cases.

Interrupt Service Routines

Interrupt service routines often set variables that are tested in mainline code. For example, a serial port interrupt may test each received character to see if it is an ETX character (presumably signifying the end of a message). If the character is an ETX, the ISR might set a global flag. An incorrect implementation of this might be:

With compiler optimization turned off, this code might work. However, any half decent optimizer will "break" the code. The problem is that the compiler has no idea that etx_rcvd can be changed within an ISR. As far as the compiler is concerned, the expression !ext_rcvd is always true, and, therefore, you can never exit the while loop. Consequently, all the code after the while loop may simply be removed by the optimizer. If you are lucky, your compiler will warn you about this. If you are unlucky (or you haven't yet learned to take compiler warnings seriously), your code will fail miserably. Naturally, the blame will be placed on a "lousy optimizer."

The solution is to declare the variable etx_rcvd to be volatile. Then all of your problems (well, some of them anyway) will disappear.

Multithreaded Applications

Despite the presence of queues, pipes, and other scheduler-aware communications mechanisms in real-time operating systems, it is still fairly common for two tasks to exchange information via a shared memory location (that is, a global). Even as you add a preemptive scheduler to your code, your compiler has no idea what a context switch is or when one might occur. Thus, another task modifying a shared global is conceptually identical to the problem of interrupt service routines discussed previously. So all shared global variables should be declared volatile. For example, this is asking for trouble:

This code will likely fail once the compiler's optimizer is enabled. Declaring cntr to be volatile is the proper way to solve the problem.

Final Thoughts

Some compilers allow you to implicitly declare all variables as volatile. Resist this temptation, since it is essentially a substitute for thought. It also leads to potentially less efficient code.

Also, resist the temptation to blame the optimizer or turn it off. Modern optimizers are so good that I cannot remember the last time I came across an optimization bug. In contrast, I come across failures by programmers to use volatile with depressing frequency.

If you are given a piece of flaky code to "fix," perform a grep for volatile. If grep comes up empty, the examples given here are probably good places to start looking for problems.

This article was published in the July 2001 issue of Embedded Systems Programming. If you wish to cite the article in your own work, you may find the following MLA-style information helpful:

How do we use volatile with structs and pointers to structs? Do we define the pointer to the struct as volatile, or do we define the individual elements of the struct each as volatile or not? Maybe a peripheral version code register doesn't need volatile as that value should never change, but status, interrupt mask, receive buffer, etc. values will?

Although I am aware of the use of volatile. But I never practically experience the affect cause when the variable doesn't contain volatile keyword. I never use any RTOS but in normal interrupt ISR and memory mapped peripheral even a normal variable yield the same result as that of volatile defined variable.

The compiler I am using is KEIL and CPU is ARM7TDMI LPC2468.

Can anyone suggest me how to visualize the effect of with/without volatile.

Hi,
If you still need this answer.
One of a few reasons for volatile.
If you have a global that is modified inside an ISR, you want to make sure that outside the ISR it is read each time it is tested. Since int_a is not modified inside the while loop the compiler may optimize the code so that the value is stored in a register and not re-read. In the following code without the volatile the while loop may never break out. This is compiler dependent. It also depends on the complexity of the code. Look at the assembled code. Change optimizer settings and recompile. Look again for the assembly code changes. Good practice ...use volatile.

Hi, you said, "Volatile pointers to non-volatile data are very rare (I think I've used them once), but I'd better go ahead and give you the syntax."
When do you use a volatile pointer to non-volatile data? I'm thinking that I need one for just this case: I have a C++ class where a user can attach their own function that they want to be called periodically inside an interrupt. I do this via a function pointer. The function pointer needs to be volatile, since it can be written outside the ISR, but is read inside the ISR, but the data (function itself) does not need to volatile. Is this correct?